Unfortunately, we don't know which retailers yet—only that they're "well-known" and common in malls—but the perpetrators might be the same dudes who hacked Target and Neiman Marcus, which experienced a similar breach in December.

Target's not alone in its credit card hacking woes: this week, high-end retailer Neiman Marcus …
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According to the report, the hackers likely hail from Eastern Europe—and it turns out they staged a series of "trial" hacks that took place in late November. Those attacks were used to help plan the larger operation in December, using a technique called "RAM scraping:"

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One of the pieces of malware they used was something known as a RAM scraper, or memory-parsing software, which enables cyber criminals to grab encrypted data by capturing it when it travels through the live memory of a computer, where it appears in plain text, the sources said.

Legal snarls complicate informing customers, and credit card companies aren't allowed to disclose the names of hacked companies until they do so themselves. It sounds as though these new breaches might not be as gargantuan as Target's, which is good. But counting Target and Neiman Marcus alone, the info of roughly 110 million customers has already been stolen. And that number is set to make a huge jump as the Department of Justice investigates these new breaches.